Fluid delivery systems are widely used. An example of a fluid delivery system is a blood pump system. The human heart can become damaged or dysfunctional over time. When damage to the heart becomes sufficiently serious, the heart fails to pump and circulate blood normally, resulting in a condition known as heart failure. Around the world millions of people suffer from heart failure. Many people are unresponsive to pharmacological intervention and could benefit from a heart transplant. However, there is a shortage of donor hearts. As a result, implantable blood pumps have gradually evolved into a viable treatment option.
In a diseased state, one or both of the ventricles of the heart can become greatly weakened to an extent that mechanical intervention is needed to keep a patient alive. In extreme circumstances, the entire heart is removed and replaced with an artificial heart while in other cases a heart assist device is used. A blood pump system used without removing the natural heart is commonly referred to as a ventricular assist device.
Although either of the ventricles of the heart may function in a weakened state, failure of the left ventricle is more common. Normally, blood enters the left ventricle through the mitral valve and, during heart systole, the blood is ejected through the aortic valve and into the aorta by the squeezing action of the left ventricle. To assist a failing left ventricle, an implantable ventricular assist device can be attached to the apex of the left ventricle supplementing blood flow between the left ventricle and the aorta. As a result, blood entering the left ventricle may either be ejected through the aortic valve by the ventricle or pass through the ventricular assist device and into the aorta.
Ventricular assistance has been performed by a variety of blood pump designs. The majority of the early ventricular assist devices, such as positive displacement pumps, pumped blood in a pulsatile manner. In this case, the ventricular assist device has allows an internal sac to passively fill with blood and then utilizes pneumatic action to compress the internal sac, ejecting the blood into the patient's aorta to supplement circulation. These pulsatile ventricular assist devices are large and can only be used as an implantable treatment option for patients with a large body surface area.
To overcome the size and reliability problems associated with the pulsatile ventricular assist devices, designers have begun to use continuous flow pumps. These pumps are smaller than their pulsatile counterparts and are more reliable. Continuous flow pumps are normally either centrifugal flow pumps or axial flow pumps. In the centrifugal flow pumps, the rotors are shaped to accelerate the blood circumferentially and thereby cause it to move toward the outer rim of the pump, whereas in the axial flow pumps the rotors are cylindrical with helical blades, causing the blood to be transported in the direction of the rotor's rotational axis.
One problem that can occur with a blood pump is that thrombus forms in the pump or is ingested, causing pump occlusion. Pump occlusion can create a number of problems. For example, pump occlusion can restrict blood flow through the pump, causing blood flow errors and disruptive blood flow conditions. Furthermore, pump occlusion causes resistive forces that reduce the pump system's overall efficiency.
One complicating factor in troubleshooting pump occlusion is that a patient's physiological conditions can also increase pump pressure and reduce pump flow. Such physiological conditions may include, for example, a restriction of the patient's peripheral vascular system. To differentiate between pump occlusion and patient's physiological conditions may be difficult and often requires the use of echocardiography. In addition, sensors, such as flow meters and pressure transducers, have been incorporated into blood pumps to help differentiate the different conditions and monitor the system. However, flow meters and pressure transducers add to the complexity, size and cost of the blood pump system, and also add complexity to the surgical procedure for implanting the blood pump system. In addition, flow meters and pressure transducers could be encapsulated or coated with biological materials and tissues can grow onto the sensing surfaces, rendering the flow meters and pressure transducers unfit for long-term use.
Another problem associated with pump thrombus is that it makes estimating pump flow rate more difficult and less accurate. Methods for estimating the flow rate of a blood pump without the use of a flow meter or pressure transducer have been suggested. For example, the parameters of an electric motor that drives a blood pump can be used to estimate the flow rate of the blood pump. However, these methods are not reliable when thrombus forms in a blood pump.
In summary, available methods for monitoring pump occlusion are complex, large, costly, and in some cases unreliable.